In a report released in April, UNESCO "noted with concern" proposals to dump three million cubic meters of dredge spoil inside the World Heritage area from the Abbot Point site.

It said it also "regrets" the approval for the dredge spoil to be dumped "prior to (the government) having undertaken a comprehensive assessment of alternative and potentially less impacting development and disposal options."

In a statement following the Committee's extension of the deadline on Wednesday, Minister Hunt defended the decision, saying the approval of the Abbot Point development "complied with Australia's obligations under the World Heritage Convention, was subject to rigorous environmental assessment and applied Australia's world-leading net benefit approach."

'Years of neglect'

The degradation of the Great Barrier Reef has been occurring for decades, according to scientific reports and anecdotal evidence.

In a speech on Tuesday in Washington, D.C., actor and director Leonardo DiCaprio declared he'd seen the "devastation" firsthand.

"What once had looked like an endless underwater utopia is now riddled with bleached coral reefs and massive dead zones," he said, comparing his experience diving on the Great Barrier Reef 20 years ago to a dive at the same location two years ago.

The same year, in 2012, the then environment minister Tony Burke admitted that years of neglect had contributed to a sharp drop in the reef's coral coverage.

"There's no doubt there's been a level of neglect for decades which if it had been dealt with otherwise we'd be in a much better situation now," Burke told the Australian Broadcasting Corporation, at the time.

He was responding to a report from the Australian Institute of Marine Science (AIMS) and the University of Wollongong that the reef had lost half of its coral cover in the last 27 years.

Most of the damage was inflicted by tropical cyclones, the report concluded, as well as coral predation by the crown-of-thorns starfish and coral bleaching, mostly caused by warmer water.

Battered by cyclones

Images released this week by the Catlin Seaview Survey show the devastating effects of Cyclone Ita, a category five tropical cyclone, which hit the region in April.

Before and after images show colorful coral snapped and flattened by waves whipped by fierce winds, which smash reefs, especially those not far from the surface.

"There is a myriad of increasing and cumulative impacts on the Great Barrier Reef. We can control impacts such as fishing, coastal development and marine debris. However we do not have the same control over storms and cyclones," said Ove Hoegh-Guidberg, Catlin Seaview Survey's chief scientist.

"Ensuring that our coral can bounce back from these disturbances is at the heart of the matter," he said.

The government says it's "absolutely committed" to protecting the Great Barrier Reef and is spending around $180 million dollars each year to improve the reef's health.

However, it remains to be seen whether it's enough to protect what the environment minister has in the past called a "vast and priceless ecosystem."